There is an apparent incompatibilty between the Norton Toolbar and the new beta of the Google Toolbar (with Sidewiki, version 6.1.20090917Wb).
If you install the new version of the toolbar, it will result in the message 'The URL is not valid and cannot be loaded' on every Firefox startup. Until Symantec and Google get in touch and resolve this issue, I would recommend sticking with the older version of the toolbar (version 5.0.20090813W), especially if you are an easily irritable type of person.
This is also a notice for Symantec to get in touch with the Google toolbar team and ask them: 'So, what's up with our toolbar(s) then?'
If someone has already updated and doesn't know how to get back to the old version, you should be able to get it here.